The reduction of nitrogen oxide (“NOx”) emissions from industrial, commercial and small electric utility boilers, gas turbines, and other lean burn stationary combustion sources continues to be a challenge. Primary measures, such as low NOx burners, flue gas recirculation, water injection, fuel staging or air staging, need to balance the impact on the efficiency and stability of combustion with the level of NOx reduction obtained and the risk of increases in other regulated pollutants, such as carbon monoxide or unburned hydrocarbons. Secondary measures, including selective non catalytic reduction (SNCR) and selective catalytic reduction (SCR), involve the injection of reagents, such as ammonia or urea, into the upper furnace or the flue gases to chemically convert NOx to elemental nitrogen.
Ammonia reagent is regulated as a hazardous substance, which has driven many end users to consider aqueous urea reagent as an alternative. While aqueous urea is not a hazardous substance, its application for NOx reduction requires additional design effort to make certain that the urea is fully gasified and does not leave intermediate solid by products that can foul surfaces and reduce chemical utilization.
While injection of urea into the cavity formed between the second and third pass of a fire tube boiler has been demonstrated to provide conversion of urea to ammonia for SCR, as described in U.S. Patent Application Publication Nos. US 2012/0177553 A1 and US 2013/0152470 A1 (both titled “Injector and Method for Reducing NOx Emissions from Boilers, IC Engines and Combustion Processes”), the injection of urea directly into the furnace of some larger industrial and small utility boilers for SCR applications is not practical due to the tight tube spacing in the furnace convective zone which prevents adequate distribution of the reagent into the furnace gases. Urea deposition on boiler tube surfaces and corrosion of water wall surfaces in the boiler is also a concern with direct injection into the furnace convective zone. SNCR technology, while allowing for injection into the high temperature combustion zone of a furnace at typical temperatures of 1700° F.-2100° F., suffers from poor reagent utilization and lower NOx reduction rates of only 20-45% versus 80-95% for well designed SCR systems.
There have been several attempts to overcome the disadvantages of known urea based NOx reduction systems. For example, U.S. Pat. No. 7,815,881 to Lin et al. describes the use of a flue gas bypass duct for injection of urea and for conversion to ammonia for SCR. U.S. Pat. No. 7,090,810 to Sun et al. describes the reduction of NOx from large-scale combustors by injecting urea into a side stream of gases with temperature sufficient for gasification and a residence time of 1-10 seconds.
However, the patents of Lin and Sun appear directed at large utility boilers. Utility boilers normally have sufficient heat input, flue gas temperatures and furnace residence times to generate 50 MW or more of electric power and are typically rated at 100 MW-800 MW or more. Whereas most industrial commercial boilers are rated below 300 million Btu/hour heat input, or roughly 30 MW equivalent.
Additionally, U.S. Pat. No. 5,296,206 to Cho et al. describes a process directed at large utility boilers, which achieves reagent flow rates up to 3,000 lbs/hr using a heat exchanger disposed in the flue gas pass such that a heated transfer medium is used to vaporize an aqueous reducing agent, which is preferably aqueous ammonia. However, Cho requires the use of a separate vaporizer vessel where the aqueous solution and heated air are mixed at the top of the vessel and the preferred outlet temperature is 250° F.-500° F. The vaporization vessel of Cho represents an additional expensive piece of equipment that can be prone to plugging from the incomplete decomposition of urea, especially at the described low exit temperatures of 250° F.-500° F. described by Cho.
Due to their smaller size and generally lower baseline NOx emissions, the cost per ton of pollutant removed from an industrial boiler or small utility boiler can be extremely high when the capital intensive control technologies such as those of Sun, Lin and Cho, which are designed for large utility boilers, are applied to small utility or industrial and commercial boilers.
Other commercial processes for the conversion of urea to ammonia involve the use of supplemental heaters, burners or high temperature steam to provide heat for conversion of urea to ammonia and they often involve a separate storage vessel to hold the ammonia gas. U.S. Pat. No. 6,436,359 to Spencer and U.S. Pat. No. 6,322,762 to Cooper generally describe generating ammonia by heating urea under pressure. These systems can be complicated to control, require additional power to operate the heaters and are expensive relative to the cost of a small industrial or commercial boiler.
U.S. Pat. Nos. 5,968,464 and 6,203,770 to Peter-Hoblyn et al. describe the proposed conversion of urea to ammonia in the exhaust of a diesel engine by injecting urea onto the heated surfaces of a pyrolysis chamber mounted in the exhaust. The pyrolysis chamber is presented in the figures and described as a foraminous structure of sintered metal, glass or ceramic material inserted in the flue gas such that when urea is injected into the structure it is converted to ammonia which then exits the foraminous structure and mixes in the flue gas. However, this structure will quickly plug with unconverted urea byproducts. In U.S. Pat. No. 6,361,754 to Peter-Hoblyn et al. it is described to convert the urea solution to ammonia by injecting the urea into a heated line disposed within an exhaust pipe, with an optional heated vessel, and then releasing ammonia through a valve mechanism into the exhaust gases upstream of an SCR reactor. However, urea solution pumped into a small heated line would be prone to plugging of the line from urea decomposition products, which would present significant resistance to the continuing flow of urea solution through the line.
A further problem with all of the prior art systems and methods described above that employ bypass or slipstream ducts for ammonia to urea conversion relates to the standard use of particulate control devices (such as mechanical separators, bag houses, etc.) in connection with modern boilers firing coal, oil and bio mass fuels. The exhaust gases are hottest immediately downstream of the combustion chamber of the boiler, and it is desirable to employ this heat as part of the ammonia to urea conversion, so that any supplemental heating required can be kept low. Thus, it is desirable to position the inlet to the bypass or slipstream duct close to the combustion chamber of the furnace (and prior to downstream components such as economizers and air heaters that cool the exhaust gas).
However, when this is done, a question arises as to where to position the outlet of the slipstream, after the urea has been injected therein. If the outlet for the slipstream is positioned in the vicinity of the inlet (i.e., close to the combustion chamber of the boiler), the gasified ammonia will pass through the downstream components, including downstream particulate control devices (such as mechanical separators, bag houses, etc.). This may be problematic, since the gasified ammonia passing through the particulate control devices may react with solids and gaseous species in the particulate control devices to form byproducts that can foul the particulate control devices or which can act to remove the ammonia from the gas stream prior to reaction across an even further downstream SCR catalyst, thereby reducing the efficiency of the process.
On the other hand, if the outlet for the slipstream is positioned downstream of the particulate control devices (such as mechanical separators, bag houses, etc.), the problem arises that the exhaust gas passing through the slip stream has never flowed through the particulate control devices, such that the particulates bypassing the particulate control devices may foul the ammonia injection grid (AIG), SCR catalyst and/or be released into the atmosphere, thereby reducing the efficiency of the process.
Thus, according to known designs, one of the following is generally true: (1) the inlet of the slipstream or bypass duct in which urea is converted to ammonia is positioned downstream of the particulate control devices, meaning that a significant portion of the heat of the exhaust gas leaving the combustion zone of the boiler is not used in the urea to ammonia conversion; (2) the inlet of the slipstream or bypass duct in which urea is converted to ammonia is positioned in close proximity to the combustion zone of the boiler and the outlet of the slipstream is positioned upstream of the particulate control devices, meaning that undesirable reactions of the ammonia within the particulate control devices may take place; or (3) the inlet of the slipstream or bypass duct in which urea is converted to ammonia is positioned in close proximity to the combustion zone of the boiler and the outlet of the slipstream is positioned downstream of the particulate control devices, meaning that some of the exhaust gases bypass the particulate control devices such that particulates may reach the AIG, SCR catalyst and or the stack.
Therefore, what is desired is a boiler employing a slipstream or bypass duct of exhaust gas in which urea is converted to ammonia, which takes advantage of the heat present in the primary exhaust stream exiting the combustion zone of the boiler in converting the urea to ammonia, and also which ensures that all of the exhaust gas is passed through the particulate control devices positioned in the primary exhaust stream.